The invention relates to a process for preparing highly pure LiBF4 in diethyl ether, and also to its use for producing lithium ion batteries.
LiBF4 can be used as a conducting salt for electrolytes in primary cells or secondary cells. In particular, it is used in rechargeable lithium ion batteries. The electrolytes are non-aqueous solutions of LiBF4 in organic media, e.g. in diethyl carbonate, dimethyl carbonate, ethylene carbonate or propylene carbonate or others, or mixtures of the solvents mentioned.
Very high requirements are placed upon the purity of the LiBF4 for this application. A particular requirement is that the LiBF4 has a very low content of free hydrogen fluoride, very low water content, and only very small amounts of foreign metal ions. Contamination with organic carbon compounds also has to be avoided.
Various methods are known for preparing LiBF4.
JP-A 61 151023 and JP-A 61 151024 describe processes in which LiF is reacted with BF3 in liquid HF to give LiBF4. Impurities then have to be removed in a second step by treatment with elemental fluorine in inert gases or in inert solvents. However, work with liquid HF and with elemental F2 is dangerous, and complicated safety precautions are therefore necessary.
Preparation in organic solvents has also been described in order to avoid preparation in liquid HF.
SU 1013405 describes the preparation of LiBF4 in tetrahydrofuran, in which LiBF4 has good solubility, by reacting LiF with BF3 in yields of from 86-89%. The product is isolated by concentrating the THF solution. This generally gives a product which still has considerable amounts of residual THF. To remove THF completely, drying in vacuo at from 70 to 80xc2x0 C. for from 10 to 15 hours using specific temperature programs is proposed. Procedures of this type are inconvenient and expensive.
JP-A 56 145113 discloses a process for preparing LiBF4 by reacting LiF with BF3 in non-aqueous organic solvents in which LiBF4 has good solubility and which can form complexes with BF3. Examples mentioned are tetrahydrofuran, dimethoxyethane, ethyl acetate and propylene carbonate. After the reaction of LiF with BF3, impurities are filtered off. LiBF4 is crystallized out from the filtrate by saturating the solution with BF3. With the solvent BF3 forms a complex in which LiBF4 has low solubility, and crystallizes out. Problems with this process are the complete removal of any BF3 adhering to the product and the need to use a considerable excess of BF3. Diethyl ether is not used for the process mentioned since the solubility of LiBF4 in diethyl ether at 25xc2x0 C. is only 1.3 g/100 ml (JACS, 75 (1953) 1753).
A process has also been disclosed (JACS, 75 (1953) 1753) for preparing LiBF4 in diethyl ether by reacting lithium carbonate with boron trifluoride according to the reaction equation 3 Li2CO3+8 BF3xe2x86x926 LiBF4+3 CO2+B2O3. The B2O3 formed as coproduct, and also undissolved LiBF4, are filtered off and the filtrate is concentrated to obtain LiBF4. Further amounts of LiBF4 have to be isolated by extracting the filtered-off B2O3/LiBF4 mixture with diethyl ether. However, this process is not cost-effective. Firstly, the low solubility of LiBF4 in diethyl ether means that large amounts of solvent have to be used and then evaporated, and this does not remove the requirement for an additional extraction of the residue. Besides this, some of the BF3 is converted into B2O3 coproduct.
It is an object of the present invention to provide a simple process for cost-effective preparation of highly pure LiBF4 which can be used for producing lithium ion batteries.
We have found that this object is achieved by means of a simple and cost-effective process in which, despite the low solubility of LiBF4 in diethyl ether, LiF can be reacted virtually completely with BF3 etherate and LiBF4 prepared with a high level of purity.
The invention therefore provides a process for preparing highly pure LiBF4 by preparing a solution of BF3 etherate in diethyl ether, suspending LiF in this solution, reacting to give solid LiBF4, and then separating off the solid LiBF4 formed.
The novel process uses BF3 etherate. The BF3 etherate is preferably diluted with diethyl ether. The solution of BF3 etherate in diethyl ether can be prepared by diluting pure BF3 etherate with diethyl ether. It is also possible to pass the desired amount of gaseous BF3 into diethyl ether. The molar ratio of diethyl ether to BF3 etherate is preferably from 0.1 to 3, particularly preferably from 0.5 to 1.5.
Finely divided LiF is added to this solution and suspended in the solution. The LiF is advantageously ground before addition. However, it is also possible to suspend LiF in diethyl ether and to add this suspension to pure BF3 etherate or to a solution of BF3 etherate in diethyl ether, or to pass BF3 gas into the suspension. The amount of BF3 etherate used is at least stoichiometrically equivalent to that of LiF. It is advantageous to work with a small excess of BF3 etherate, but the molar ratio of BF3 etherate to LiF should not be greater than 1.2. The molar ratio of BF3 etherate to LiF is preferably from 1 to 1.1.
The reaction is generally carried out with heating, preferably heating to reflux. The reaction time depends on the temperature selected. The reaction is generally allowed to continue for from 4 to 10 h. However, it is also possible to work at room temperature with a corresponding increase in reaction time.
The suspending of LiF and the reaction to give LiBF4 are advantageously carried out with intensive mixing, for example with the aid of stirrers or of dispersing equipment. Mixing devices and apparatuses of this type are known per se.
The LiBF4 obtained from the reaction, substantially suspended in diethyl ether, is separated off by processes known per se, such as filtration, pressure filtration, centrifuging or decanting, and dried. Before filtering off it is advantageous to cool the suspension at least to room temperature.
Surprisingly, in spite of the low solubility of LiBF4 in diethyl ether, the suspended LiF reacts virtually completely with the BF3 to give LiBF4. The proportion of LiF in the final LiBF4 product is not more than 2% by weight, generally less than 1%. The final product also has only low proportions of acid impurities, low moisture and low proportions of heavy metals. The highly volatile diethyl ether is easily removed from the final product. The residual proportion of organic carbon (total organic carbon, TOC) is less than 0.1% by weight.
This purity gives the LiBF4 prepared by the novel process excellent suitability for use as a conducting salt in lithium ion batteries.